On Erik’s death in 1250 Birger’s son Valdemar[?] was elected king while his father acted as regent. During the sixteen years of his sway Sweden advanced greatly in fame and prosperity. In 1249 he led an expedition to Finland, built the fortress of Tavastehus, and thus laid the foundations of Sweden’s oversea empire. He also built Stockholm, and enriched it by making it the chief mart for the trade of Lübeck, with which city he concluded a commercial treaty. As a lawgiver Birger laboured strenuously in the interests of civilization.